This invention relates to golf club heads of the iron type, and, more particularly, to an iron club head which is provided with a rear cavity which is shaped to position a substantial portion of the weight of the club head high in the toe and low in the heel of the club head.
It is commonly assumed that when a golf club strikes a golf ball off center, i.e., at a point spaced from the center of gravity of the club toward the toe or heel, the club rotates about a vertical axis which extends through the center of gravity. When the club strikes the ball at a point above or below the center of gravity, i.e., toward the top or sole of the club, it is assumed that the club tends to rotate about a horizontal axis which extends through the center of gravity. These assumptions have resulted in clubs being designed with increased polar moments of inertia about the vertical and horizontal axes in order to reduce the rotation of the club. The moment of inertia about the vertical axis may be increased by concentrating weight of the club in the toe and the heel. The moment of inertia about the horizontal axis may be increased by concentrating weight of the club in the sole and the top.
U.S. Pat. No. 4,471,961 states that a golf club head does not rotate about a vertical axis on toe or heel hits and does not rotate about a horizontal axis on high and low hits. Rather, a toe or heel hit will cause the head to rotate about a first axis which extends through the center of gravity of the club head in a plane which intersects the axis of the shaft above the club head. A high or low hit will cause the club to rotate about a second axis which extends through the center of gravity perpendicular to the first axis. If the weight of the club head is distributed to increase the moment of inertia and to decrease the tendency of the club head to rotate, the weight should be distributed with respect to the actual axes of rotation. The true axis of rotation extends at about 35.degree. to about 55.degree. with respect to the horizontal.
U.S. Pat. No. 4,471,961 specifically describes a wood type club head in which the bulge and roll curvatures on the face are oriented with respect to the true axis of rotation. Column 6, lines 58-64 refer to an iron club head, but the patent does not describe any specific means for orienting the weight of the club head.
A club head formed in accordance with the invention is provided with a cavity in the back of the club head which has a pair of substantially straight, substantially parallel side walls which extend at an angle from about 0.degree. to about 50.degree. to the hosel of the club head. The resulting shape of the cavity causes a substantial portion of the weight of the club head to be distributed in the upper portion of the toe of the club head and in the lower portion of the heel of the club head. The club head therefore has a significant polar moment of inertia about the true axis of rotation, i.e., an axis which extends through the center of gravity at an angle of about 35.degree. to about 55.degree. to the horizontal.